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I'm trying to define what a small DC motor would be. I'm having problems why some kind of DC motors can't be built in small size.

Look this diagram:

enter image description here

I'd like to know why some of these kinds of motors can't be small, I mean, in constructive ways.

When I say small, I mean motors like this one:

enter image description here

Thank you!

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    \$\begingroup\$ Well, the permanent magnet is probably going to be more compact than a coil, is very simple, etc. \$\endgroup\$
    – Transistor
    Jun 19, 2016 at 22:34
  • \$\begingroup\$ Got it! Thank you! But are there only permanent magnet small motors? \$\endgroup\$ Jun 19, 2016 at 22:36

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It is all about cost and benefits. DC motors larger than a few thousand watts are nearly extinct because the same or better performance can be provided at an equal or lower cost considering the motor, the required controls and operating costs. Separately excited DC motors with electronic speed controls (ESCs) were widely used from the late 1950's until the 1990's. By that time AC motors with variable frequency drive controls (VFDs) were being used for the majority of larger applications. Now, permanent-magnet DC motors have become the most economical alternative for many smaller applications both with and without ESCs. There are still many series-wound motors used in small appliances, but permanent magnet motors with or without ESCs are finding their way into those applications.

Series DC motors have probably been manufactured in sizes close to the motor that you show in your question. They are the least expensive very small motor to manufacture except for permanent magnet DC motors. As long as suitable magnetic materials have been available, these types of motors have mostly been permanent magnet motors.

There is also a variety of permanent-magnet and reluctance synchronous AC motors that have been widely used in such applications as electromechanical clocks and timers, small aquarium pumps, and microwave oven food rotators.

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Wikipedia's DC motor article may answer the question in a backwards kind of way. (Emphasis mine.)

A PM motor does not have a field winding on the stator frame, instead relying on PMs to provide the magnetic field against which the rotor field interacts to produce torque. Compensating windings in series with the armature may be used on large motors to improve commutation under load. Because this field is fixed, it cannot be adjusted for speed control. PM fields (stators) are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings. Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed amount of flux. However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines.

To minimize overall weight and size, miniature PM motors may use high energy magnets made with neodymium or other strategic elements; most such are neodymium-iron-boron alloy. With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed singly fed synchronous and induction electric machines.

It suggests to me that permanent magnet would be used for all sizes were they not so awkward to deal with during assembly / disassembly.

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    \$\begingroup\$ I thought large powerful permanent magnets are somewhat fragile as the magnetic field tends to 'rip themselves apart'. They are potential shrapnel grenades. So it isn't just assembly/disassembly which adds a bit of complexity. While I don't think it is significant, very large permanent magnets might be a bit awkward for people in their locale. MRI machines effectively have large permanent magnets, and you have to be careful if you have bits of metal in your body, and they can damage watches or equipment with small moving parts. \$\endgroup\$
    – gbulmer
    Jun 19, 2016 at 23:27

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